When two components are connected by means of bonding, gluing or welding, the elasticity of the components, namely their ability to bend or yield to pressure, often constitutes a significant problem in terms of the quality of the connection. The quality and integrity of the connection can be reduced, particularly in the case of extrusion-blown hollow plastic bodies, due to the inconstant positioning of one of the components during the melting and pressing process. Leaky weld connections can lead to serious consequences in the context of applications where weld connections of a high quality are required, as for example in the manufacture of plastic fuel tanks which require high tolerance limits as far as the location of the connecting surfaces is concerned. In the context of such applications, the compensation of tolerance limits for the positioning of the contact surfaces with help of a welding system is required.
German Patent No. DE 35 37 670 C2 to Oxenfarth, which issued on Mar. 3, 1988, discloses a conventional process and corresponding device for use in forming a plastic weld connection. A minimum-force sensor mechanically determines the position of the contact surface on a workpiece such as a hollow plastic container (e.g. a fuel tank of a vehicle). Once the workpiece position is determined, a welding head or other heat applying device is moved by the sensed distance into contact with the workpiece to soften and melt a portion to which a second component is to be welded. This process assures safe welding of the second component, which for example may be a vent connection or filler neck onto the workpiece in spite of high tolerance limits as far as the dimensions of the workpiece are concerned. Therefore, the use of thicker materials to ensure complete welds, and which is almost impossible in the case of thin-walled hollow bodies, is not required to effect the connection. This procedure, however, neglects the forces that melting head apply to the workpiece and which can cause distortion or bending of workpiece surfaces, leading to incomplete welds.
DE-PS 39 22 066 discloses a further development of the state of the art used to achieve improved welds. This reference discloses a method to compensate for position changes and deformation of the part of a workpiece to which a second component is to be connected by readjusting the position and contact pressure of a heating element, as well as the position and contact pressure of the second component to be connected to the workpiece. For this purpose, the position of the contact surface on the workpiece is also determined by a minimum-force mechanical sensor. The result of the measuring process is then used to readjust a piston which presses the component to be added against the heating device and the heating device against the workpiece.
Published Japanese Patent Abstract No. 04 234633 to Yokoyama Takahito (published Aug. 24, 1992) discloses a holder used to position a polyethylene joint on a tank, following melting and heating of contact surfaces. The holder includes a compression spring used to bias a melted surface of the joint against a melted surface of the tank.
Published Japanese Patent Abstract No. 08 112861 to Imagawa Kanji (published May 7, 1996) discloses a heating device used to heat weld together two sheets. The heating device includes a heating part which is biased by means of a spring to project downwardly through an opening, into contact with the sheets which are to be welded.
The determination of the position of the workpiece surface using an additional sensing device constitutes a disadvantage in that it is not sufficiently accurate, as the method requires the establishment of additional reference surfaces on a potentially elastically deformable surface of the workpiece or component. In addition, the reference surfaces are often not feasible for manufacturing related reasons, or are located too far from the relevant connecting position, thus distorting the result.
Plastic welding units that are exclusively equipped with load-sensitive sensors such as load cells can compensate for the disadvantages of external sensing equipment but are often expensive and are not sufficiently sensitive for design-related reasons. Additionally, the impression depth achieved by the partial melting of the component or workpiece surface is not adequately determined.